The panel placed below the vertical sign...
A)warns of the presence of a sign.
B)warns the driver of adverse conditions such as snow, fog or rain.
C)is supplementary to the vertical sign and indicates fog.Correct
The supplementary panel in the question indicates, beneath another vertical sign, that the sign applies to the circumstances shown on the panel, in this case fog. For example, the following signage advises a maximum speed of 70 kilometres per hour only in the event of fog.
Specific panels also exist for snow and rain.